Why is that? (this is not a criticism, but a question). Did they have a treaty?
Not at all. The Dutch were pointedly neutral.
Now, of course, it is in the French and British interests to prop the DEIs up against a Japanese invasion, so it's entirely possible that some way to help the Dutch comes up.
In particular, one of the possible starting scenarios is that the Germans have been defeated in 1940. If so, the Dutch are de facto allied with the British and the French, and they may well be the ones who ask for help against another enemy.
Alternately, the British in particular seem to have been inclined, at this time, to proffer unilateral guarantees to countries not their allies, if they could be used as a tripwire against a major threat. Maybe the Japanese try first with some saber-rattling, the Dutch don't know what to do, and the British have time to offer them a guarantee.
Finally, there's British and French warships steaming around the place, as well as US ones, all astride the Japanese LOCs to the DEIs. It only takes a more or less unintended firefight at sea to provide a casus belli, if one feels it is in his country's interest to tackle the Japanese.